1. Field of the Invention
The present invention relates to security for communication systems and more specifically to transmission level security for communication systems using multiple carrier modulation techniques. Even more specifically, the present invention relates to transmission level security for communication systems using multiple carrier modulation techniques, such as orthogonal frequency division multiplexing (OFDM).
2. Discussion of the Related Art security is becoming increasingly important in communications systems which transmit data. Wireline communication systems, such as coaxial cable and especially fiber optic cable, are inherently secure since such mediums must be physically tapped to intercept data transmissions. On the other hand, wireless data transmission is inherently insecure, since data is transmitted openly over an air interface such that an unauthorized entity may easily intercept the transmission. In the context of wireless local area network (LAN) applications, security is extremely important since sensitive data is transmitted to and from multiple devices that constitute the network. Without any security measures, eavesdroppers may freely receive data transmissions between devices in the wireless LAN. As such, transmission security is required to ensure that unauthorized entities are not allowed to intercept the data as it is communicated across the network.
Many bit-level security technologies exist in which the data is encrypted prior to transmission over the wireless medium. Typically, encryption involves scrambling the data bits prior to transmission and the receivers in the network are provided with the logic to decrypt the transmitted information. Common bit-level security techniques include DPF (Data Private Facility) and DES (Data Encryption Standard). These encryption techniques make use of either a Public Key or a Private Key which is used by authorized receivers to decrypt the data. As such, without the key, unauthorized users have a difficult time descrambling the data.
Many bit-level security measures, for example, those used in OFDM (Orthogonal Frequency Division Multiplexed) wireless LAN applications using the HiperLAN2 and IEEE802.11 standards, offer data payload level encryption to encrypt the data but transmit the preambles and control fields with no protection. Thus, an unauthorized eavesdropper can easily intercept the data, although it is encrypted. Disadvantageously, having received the data, the unauthorized eavesdropper can easily parse the data fields from the PHY-layer communications and possibly reverse engineer the encryption technique and extract the underlying source information.
Another form of security is transmission-level security in which the physical waveform (i.e. PHY-layer) carrying the data is altered according to an encryption technique. Advantageously, since the physical waveform is altered, unauthorized eavesdroppers may not even be able to meaningfully receive the transmitted signal at all. One conventional transmission-level encryption technique includes frequency hopping in which the carrier frequency that carries the data is periodically hopped between several different frequencies in a pseudo random fashion. Unless an eavesdropper is aware of the frequencies used and the hopping pattern, the eavesdropper will be unable to receive the transmitted data.
The present invention advantageously addresses the needs above as well as other needs by providing a transmission level security technique which introduces a known group delay distortion into the transmitted signaling, which for the unequipped receiver, will interfere with the reception of data and in some embodiments, interfere with the detection and acquisition of the preamble for the frame carrying the data.
In one embodiment, the invention can be characterized as a method of transmission level security in a communication system, and a means for accomplishing the method, the method including the steps of: forming a plurality of digital signals representing a symbol to be transmitted over a communication medium, wherein respective ones of the plurality of digital signals are modulated onto respective ones of a plurality of subcarriers according to a multiple carrier modulation scheme; and introducing a group delay distortion in one or more of the plurality of subcarriers, wherein a peak-to-peak variation of the group delay distortion is greater than a guard time interval corresponding to the symbol, such that portions of the one or more of the plurality of subcarriers will be received outside of a time window corresponding to the symbol at a receiver.
In another embodiment, the invention can be characterized as a method of transmission level security in a communication system including the steps of: forming a signal to be transmitted over a communication medium; and introducing a group delay distortion in the signal, wherein the group delay distortion will cause sufficient signal energy to be dispersed in time outside of a nominal window of time corresponding to a signal feature of the signal at a corresponding receiver, wherein frequency bin splattering will occur in a Fourier transform of the receiver.
In a further embodiment, the invention can be characterized as a method of transmission level security for a communication system including the steps of: receiving a signal comprising a plurality of subcarriers representing a symbol and having been transmitted according to a multiple carrier modulation scheme, wherein a respective one or more of the plurality of subcarriers has undergone a predetermined group delay distortion, wherein a peak-to-peak variation of the delay of the predetermined group delay distortion is greater than a guard time corresponding to the symbol such that portions of the one or more of the plurality of subcarriers would fall outside of a time window associated with the symbol; and removing the predetermined group delay distortion from the one or more of the plurality of subcarriers such that all of the plurality of subcarriers fit within the time window associated with the symbol.
In an additional embodiment, the invention can be characterized as a method of transmission level security for time windowed communication systems including the steps of: receiving a signal, wherein the signal has been undergone a predetermined group delay distortion, wherein the predetermined group delay distortion is such that sufficient signal energy will be received outside of a nominal window of time corresponding to a signal feature of the signal such that will cause frequency bin splattering in a Fourier transform; and removing the group delay distortion from the signal such that the signal fits within the nominal window of time.
In an added embodiment, the invention can be characterized as a transmitter implementing transmission level security for wireless communications including a baseband modulator producing digital baseband signals representing data symbols and modulated on a plurality of subcarriers and a time dispersive all-pass filter coupled to the baseband modulator. The time dispersive all-pass filter applies a group delay distortion upon a respective one or more of the plurality of subcarriers. A peak-to-peak variation of the group delay distortion is greater than a guard time corresponding to a time window of each of the data symbols such that a receiver will receive portions of the one or more of the plurality of subcarriers outside of the time window.
In a supplementary embodiment, the invention can be characterized as a method of transmission level security in a communication system including the steps of: forming an orthogonal frequency division multiplexed (OFDM) symbol comprising a plurality of subcarriers to be transmitted over a communication medium; introducing a group delay distortion in one or more of the plurality of subcarriers by passing the symbol through one or more time-dispersive all-pass filters, wherein a peak-to-peak variation of the group delay distortion is greater than a guard time interval corresponding to the symbol, such that portions of the one or more of the plurality of subcarriers will be moved outside of a nominal window of time corresponding to the symbol destroying orthogonality between respective ones of the plurality of subcarriers, whereby a receiver""s ability to retrieve the information bits from the symbol is impaired; modulating the symbol; converting the symbol to radio frequency; transmitting the symbol; receiving the symbol; converting the symbol to baseband; demodulating the symbol; and removing the group delay distortion in the one or more of the plurality of subcarriers by passing the symbol through another one or more time-dispersive all-pass filters, wherein the other one or more time-dispersive all-pass filters is the inverse of the one or more time-dispersive all-pass filters, such that each of the plurality of subcarriers will fit within the nominal window of time corresponding to the symbol and each of the plurality of subcarriers is orthogonal to each other.